Tales about him vary on important details, but he is usually said to have been born in or very near to Tregaron, in or around 1530, his mother being one Cati Jones of Tregaron. He was an illegitimate son, whom his mother named Thomas. In what is still practised in Welsh speaking communities, English names such as Thomas Jones are Cymricized, hence Twm Sion.
It is also common practice in rural Wales, being traditionally a matriarchal society, for children with common names to be nicknamed after their mothers. Thus he became known as Twm Siôn Cati.
His father was supposed to be Sion ap Dafydd ap Madog ap Hywel Moetheu of Porth-y-ffin, also near Tregaron.
He was supposedly a Protestant by faith in a time when Mary I of England, a Catholic monarch ruled and he had to gain an income as best he could, choosing robbery as his trade as his religion had him marked out as a rebel already and his lowly status meant that he couldn't rely on any advantage or protection from others. As a young man he fled to Geneva in 1550 to escape religious persecution and the law. Only the accession of Queen Elizabeth I brought him a pardon for his thievery and enabled him to return to Wales in 1559.
The original character is often said to have been based on one Thomas Jones (c. 1530-1609) who, says the Oxford Companion to the Literature of Wales, was pardoned for unspecified offences in 1559, wrote poetry, was a steward who "often had recourse to the law", and married the widow of Thomas Rhys Williams of Ystrad-ffin. It seems unlikely, however, that all the tales told of Twm Siôn Cati in later times can be attributed to this one man. The Oxford Companion further asserts "he has been confused with others of the same name who were raiders and highwaymen in the district of Tregaron" and lists another eleven Thomas Joneses in the field of literature alone.
Although the original tales were passed on orally, there were later a number of written stories of Twm Siôn Cati. An English-language pamphlet, Tomshone Catty's Tricks, was printed in 1763. William Frederick Deacon wrote two books involving him in the 1820s. In 1828, T J Llewelyn Pritchard's The Adventures and Vagaries of Twm Shon Catti, descriptive of Life in Wales was published. Enlarged (and somewhat altered) editions of this followed. An eight-page pamphlet, Y Digrifwr, was published in 1844, its subtitle admirably describing its contents ("The jokester: a collection of feats and tricks of Thomas Jones of Tregaron, Cardiganshire, he who is generally known under the name Twm Sion Catti"). George Borrow, walking through Wild Wales in 1854, heard several tales about Twm from a fellow-walker on the way to Tregaron and later read what was probably Pritchard's book.
In the tale told by Llewelyn Pritchard, Twm is the illegitimate son of Cati Jones following attentions from John Wynn of Gwydir (John "Wynn" ap Maredudd); the Welsh forms of the names of his parents became incorporated into his name. He grows up in Tregaron and after a spell working for a farmer, he works for a local landowner. He is trusted to take a large sum of the squire's money to England. The journey is fraught with encounters with highwaymen, footpads, and villains, all of whom Twm is able to best. Twm woos and eventually marries the Lady of Ystrad-ffin and subsequently becomes a magistrate and mayor of Brecon.
George Borrow disapproved of the veneer of respectability in Pritchard's book: "Its grand fault is endeavouring to invest Twm Shon (a name Borrow spells with varying consistency) with a character of honesty, and to make his exploits appear rather those of a wild young waggish fellow than of a robber." According to the stories which Borrow picked up around Tregaron, Twm's career was more straightforward. "Between eighteen and nineteen, in order to free himself and his mother from poverty which they had long endured, he adopted the profession of a thief, and soon became celebrated through the whole of Wales for the cleverness and adroitness which he exercised in his calling".
Borrow recounted a story in which a farmer is hunting Twm over the theft of a bullock. The farmer reaches Twm's mother's house and asks whether Twm Shone Catti (another of Borrow's spellings) lives there. A beggar answers that he does, and agrees to hold the farmer's horse and whip for him. As the farmer goes into the house, the beggar jumps onto the horse: it is Twm. He gallops to the house of the farmer and tells the farmer's wife that the farmer is in trouble, needs money urgently, and has sent Twm to fetch it, with the horse and whip to prove that the message really came from the farmer. The farmer's wife pays up. Twm, now in possession of the farmer's money and horse, hastily departs for London, later selling the horse.
A tale from Pritchard's book involves an occasion when Twm is staying in an inn overnight and realises other people are planning to rob him the following day after he sets off. He has a large sum of money with him. The following morning he behaves as though his money is in the pack-saddle of his horse. When the highwayman catches up, Twm drops the saddle in the middle of a pool. The highwayman wades into the pool to fetch it. Twm takes the opportunity to make off with the highwayman's horse. A complication arises because the horse responds to the voice of the highwayman crying "Stop!" Luckily Twm, in terror, happens to shout a word which makes the horse gallop again, and he is conveyed to safety.
Another tale recounts how Twm waylaid a rich squire, who was accompanied by his daughter, Twm was so smitten with her that he returned her jewellery to her and attempted to woo her, against her father's opposition and her own initially. One full moon shortly after the robbery he crept to her window, roused her from sleep, caught her hand at the window and kissed it, refusing to let her go until she promised to marry him. She wouldn't promise so Twm drew his dagger, drew blood on her wrist and threatened to sever her hand unless she assented to marriage forthwith. She agreed to marry him and she kept her hand. Their marriage followed soon after, despite her father's views and the directness of Twm's courtship methods. The girl was supposedly the widow of the Sheriff of Carmarthen. Through this marriage Twm is supposed to have gained respectability, eventually becoming a Justice of the Peace, sitting in judgement on others, a position he held until his death aged 79.